Sustained OMA1-mediated integrated stress response is beneficial for spastic ataxia type 5.

AFG3L2 is a mitochondrial protease exerting protein quality control in the inner mitochondrial membrane. Heterozygous AFG3L2 mutations cause spinocerebellar ataxia type 28 (SCA28) or dominant optic atrophy type 12 (DOA12), while biallelic AFG3L2 mutations result in the rare and severe spastic ataxia type 5 (SPAX5). The clinical spectrum of SPAX5 includes childhood-onset cerebellar ataxia, spasticity, dystonia and myoclonic epilepsy. We previously reported that the absence or mutation of AFG3L2 leads to the accumulation of mitochondria-encoded proteins, causing the overactivation of the stress-sensitive protease OMA1, which over-processes OPA1, leading to mitochondrial fragmentation. Recently, OMA1 has been identified as the pivotal player communicating mitochondrial stress to the cytosol via a pathway involving the inner mitochondrial membrane protein DELE1 and the cytosolic kinase HRI, thus eliciting the integrated stress response. In general, the integrated stress response reduces global protein synthesis and drives the expression of cytoprotective genes that allow cells to endure proteotoxic stress. However, the relevance of the OMA1-DELE1-HRI axis in vivo, and especially in a human CNS disease context, has been poorly documented thus far. In this work, we demonstrated that mitochondrial proteotoxicity in the absence/mutation of AFG3L2 activates the OMA1-DELE1-HRI pathway eliciting the integrated stress response. We found enhanced OMA1-dependent processing of DELE1 upon depletion of AFG3L2. Also, in both skin fibroblasts from SPAX5 patients (including a novel case) and in the cerebellum of Afg3l2-/- mice we detected increased phosphorylation of the α-subunit of the eukaryotic translation initiation factor 2 (eIF2α), increased levels of ATF4 and strong upregulation of its downstream targets (Chop, Chac1, Ppp1r15a and Ffg21). Silencing of DELE1 or HRI in SPAX5 fibroblasts (where OMA1 is overactivated at basal state) reduces eIF2α phosphorylation and affects cell growth. In agreement, pharmacological potentiation of integrated stress response via Sephin-1, a drug that selectively inhibits the stress-induced eIF2alpha phosphatase GADD34 (encoded by Ppp1r15a), improved cell growth of SPAX5 fibroblasts and cell survival and dendritic arborization ex vivo in primary Afg3l2-/- Purkinje neurons. Notably, Sephin-1 treatment in vivo extended the lifespan of Afg3l2-/- mice, improved Purkinje neuron morphology, mitochondrial ultrastructure and respiratory capacity. These data indicate that activation of the OMA1-DELE1-HRI pathway is protective in the context of SPAX5. Pharmacological tuning of the integrated stress response may represent a future therapeutic strategy for SPAX5 and other cerebellar ataxias caused by impaired mitochondrial proteostasis.

Translation of MT-ATP6 pathogenic variants reveals distinct regulatory consequences from the co-translational quality control of mitochondrial protein synthesis.

Pathogenic variants that disrupt human mitochondrial protein synthesis are associated with a clinically heterogeneous group of diseases. Despite an impairment in oxidative phosphorylation being a common phenotype, the underlying molecular pathogenesis is more complex than simply a bioenergetic deficiency. Currently, we have limited mechanistic understanding on the scope by which a primary defect in mitochondrial protein synthesis contributes to organelle dysfunction. Since the proteins encoded in the mitochondrial genome are hydrophobic and need co-translational insertion into a lipid bilayer, responsive quality control mechanisms are required to resolve aberrations that arise with the synthesis of truncated and misfolded proteins. Here, we show that defects in the OXA1L-mediated insertion of MT-ATP6 nascent chains into the mitochondrial inner membrane are rapidly resolved by the AFG3L2 protease complex. Using pathogenic MT-ATP6 variants, we then reveal discrete steps in this quality control mechanism and the differential functional consequences to mitochondrial gene expression. The inherent ability of a given cell type to recognize and resolve impairments in mitochondrial protein synthesis may in part contribute at the molecular level to the wide clinical spectrum of these disorders.

EDIR: exome database of interspersed repeats.

MOTIVATION: Intragenic exonic deletions are known to contribute to genetic diseases and are often flanked by regions of homology. RESULTS: In order to get a more clear view of these interspersed repeats encompassing a coding sequence, we have developed EDIR (Exome Database of Interspersed Repeats) which contains the positions of these structures within the human exome. EDIR has been calculated by an inductive strategy, rather than by a brute force approach and can be queried through an R/Bioconductor package or a web interface allowing the per-gene rapid extraction of homology-flanked sequences throughout the exome. AVAILABILITY AND IMPLEMENTATION: The code used to compile EDIR can be found at https://github.com/lauravongoc/EDIR. The full dataset of EDIR can be queried via an Rshiny application at http://193.70.34.71:3857/edir/. The R package for querying EDIR is called 'EDIRquery' and is available on Bioconductor. The full EDIR dataset can be downloaded from https://osf.io/m3gvx/ or http://193.70.34.71/EDIR.tar.gz. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

Identification of RAD17 as a candidate cancer predisposition gene in families with histories of pancreatic and breast cancers.

BACKGROUND: Among the 10% of pancreatic cancers that occur in a familial context, around a third carry a pathogenic variant in a cancer predisposition gene. Genetic studies of pancreatic cancer predisposition are limited by high mortality rates amongst index patients and other affected family members. The genetic risk for pancreatic cancer is often shared with breast cancer susceptibility genes, most notably BRCA2, PALB2, ATM and BRCA1. Therefore, we hypothesized that additional shared genetic etiologies might be uncovered by studying families presenting with both breast and pancreatic cancer. METHODS: Focusing on a multigene panel of 276 DNA Damage Repair (DDR) genes, we performed next-generation sequencing in a cohort of 41 families with at least three breast cancer cases and one pancreatic cancer. When the index patient with pancreatic cancer was deceased, close relatives (first or second-degree) affected with breast cancer were tested (39 families). RESULTS: We identified 27 variants of uncertain significance in DDR genes. A splice site variant (c.1605 + 2T > A) in the RAD17 gene stood out, as a likely loss of function variant. RAD17 is a checkpoint protein that recruits the MRN (MRE11-RAD50-NBS1) complex to initiate DNA signaling, leading to DNA double-strand break repair. CONCLUSION: Within families with breast and pancreatic cancer, we identified RAD17 as a novel candidate predisposition gene. Further genetic studies are warranted to better understand the potential pathogenic effect of RAD17 variants and in other DDR genes.

CpG Methylation, a Parent-of-Origin Effect for Maternal-Biased Transmission of Congenital Myotonic Dystrophy.

CTG repeat expansions in DMPK cause myotonic dystrophy (DM1) with a continuum of severity and ages of onset. Congenital DM1 (CDM1), the most severe form, presents distinct clinical features, large expansions, and almost exclusive maternal transmission. The correlation between CDM1 and expansion size is not absolute, suggesting contributions of other factors. We determined CpG methylation flanking the CTG repeat in 79 blood samples from 20 CDM1-affected individuals; 21, 27, and 11 individuals with DM1 but not CDM1 (henceforth non-CDM1) with maternal, paternal, and unknown inheritance; and collections of maternally and paternally derived chorionic villus samples (7 CVSs) and human embryonic stem cells (4 hESCs). All but two CDM1-affected individuals showed high levels of methylation upstream and downstream of the repeat, greater than non-CDM1 individuals (p = 7.04958 × 10-12). Most non-CDM1 individuals were devoid of methylation, where one in six showed downstream methylation. Only two non-CDM1 individuals showed upstream methylation, and these were maternally derived childhood onset, suggesting a continuum of methylation with age of onset. Only maternally derived hESCs and CVSs showed upstream methylation. In contrast, paternally derived samples (27 blood samples, 3 CVSs, and 2 hESCs) never showed upstream methylation. CTG tract length did not strictly correlate with CDM1 or methylation. Thus, methylation patterns flanking the CTG repeat are stronger indicators of CDM1 than repeat size. Spermatogonia with upstream methylation may not survive due to methylation-induced reduced expression of the adjacent SIX5, thereby protecting DM1-affected fathers from having CDM1-affected children. Thus, DMPK methylation may account for the maternal bias for CDM1 transmission, larger maternal CTG expansions, age of onset, and clinical continuum, and may serve as a diagnostic indicator.

Efficient CRISPR/Cas9-mediated editing of trinucleotide repeat expansion in myotonic dystrophy patient-derived iPS and myogenic cells.

CRISPR/Cas9 is an attractive platform to potentially correct dominant genetic diseases by gene editing with unprecedented precision. In the current proof-of-principle study, we explored the use of CRISPR/Cas9 for gene-editing in myotonic dystrophy type-1 (DM1), an autosomal-dominant muscle disorder, by excising the CTG-repeat expansion in the 3'-untranslated-region (UTR) of the human myotonic dystrophy protein kinase (DMPK) gene in DM1 patient-specific induced pluripotent stem cells (DM1-iPSC), DM1-iPSC-derived myogenic cells and DM1 patient-specific myoblasts. To eliminate the pathogenic gain-of-function mutant DMPK transcript, we designed a dual guide RNA based strategy that excises the CTG-repeat expansion with high efficiency, as confirmed by Southern blot and single molecule real-time (SMRT) sequencing. Correction efficiencies up to 90% could be attained in DM1-iPSC as confirmed at the clonal level, following ribonucleoprotein (RNP) transfection of CRISPR/Cas9 components without the need for selective enrichment. Expanded CTG repeat excision resulted in the disappearance of ribonuclear foci, a quintessential cellular phenotype of DM1, in the corrected DM1-iPSC, DM1-iPSC-derived myogenic cells and DM1 myoblasts. Consequently, the normal intracellular localization of the muscleblind-like splicing regulator 1 (MBNL1) was restored, resulting in the normalization of splicing pattern of SERCA1. This study validates the use of CRISPR/Cas9 for gene editing of repeat expansions.

GTF2E2 Mutations Destabilize the General Transcription Factor Complex TFIIE in Individuals with DNA Repair-Proficient Trichothiodystrophy.

The general transcription factor IIE (TFIIE) is essential for transcription initiation by RNA polymerase II (RNA pol II) via direct interaction with the basal transcription/DNA repair factor IIH (TFIIH). TFIIH harbors mutations in two rare genetic disorders, the cancer-prone xeroderma pigmentosum (XP) and the cancer-free, multisystem developmental disorder trichothiodystrophy (TTD). The phenotypic complexity resulting from mutations affecting TFIIH has been attributed to the nucleotide excision repair (NER) defect as well as to impaired transcription. Here, we report two unrelated children showing clinical features typical of TTD who harbor different homozygous missense mutations in GTF2E2 (c.448G>C [p.Ala150Pro] and c.559G>T [p.Asp187Tyr]) encoding the beta subunit of transcription factor IIE (TFIIEß). Repair of ultraviolet-induced DNA damage was normal in the GTF2E2 mutated cells, indicating that TFIIE was not involved in NER. We found decreased protein levels of the two TFIIE subunits (TFIIEα and TFIIEß) as well as decreased phosphorylation of TFIIEα in cells from both children. Interestingly, decreased phosphorylation of TFIIEα was also seen in TTD cells with mutations in ERCC2, which encodes the XPD subunit of TFIIH, but not in XP cells with ERCC2 mutations. Our findings support the theory that TTD is caused by transcriptional impairments that are distinct from the NER disorder XP.

Biallelic Mutations in TMEM126B Cause Severe Complex I Deficiency with a Variable Clinical Phenotype.

Complex I deficiency is the most common biochemical phenotype observed in individuals with mitochondrial disease. With 44 structural subunits and over 10 assembly factors, it is unsurprising that complex I deficiency is associated with clinical and genetic heterogeneity. Massively parallel sequencing (MPS) technologies including custom, targeted gene panels or unbiased whole-exome sequencing (WES) are hugely powerful in identifying the underlying genetic defect in a clinical diagnostic setting, yet many individuals remain without a genetic diagnosis. These individuals might harbor mutations in poorly understood or uncharacterized genes, and their diagnosis relies upon characterization of these orphan genes. Complexome profiling recently identified TMEM126B as a component of the mitochondrial complex I assembly complex alongside proteins ACAD9, ECSIT, NDUFAF1, and TIMMDC1. Here, we describe the clinical, biochemical, and molecular findings in six cases of mitochondrial disease from four unrelated families affected by biallelic (c.635G>T [p.Gly212Val] and/or c.401delA [p.Asn134Ilefs(∗)2]) TMEM126B variants. We provide functional evidence to support the pathogenicity of these TMEM126B variants, including evidence of founder effects for both variants, and establish defects within this gene as a cause of complex I deficiency in association with either pure myopathy in adulthood or, in one individual, a severe multisystem presentation (chronic renal failure and cardiomyopathy) in infancy. Functional experimentation including viral rescue and complexome profiling of subject cell lines has confirmed TMEM126B as the tenth complex I assembly factor associated with human disease and validates the importance of both genome-wide sequencing and proteomic approaches in characterizing disease-associated genes whose physiological roles have been previously undetermined.

TANGO2 deficiency as a cause of neurodevelopmental delay with indirect effects on mitochondrial energy metabolism.

Exome sequencing has recently identified mutations in the gene TANGO2 (transport and Golgi organization 2) as a cause of developmental delay associated with recurrent crises involving rhabdomyolysis, cardiac arrhythmias, and metabolic derangements. The disease is not well understood, in part as the cellular function and subcellular localization of the TANGO2 protein remain unknown. Furthermore, the clinical syndrome with its heterogeneity of symptoms, signs, and laboratory findings is still being defined. Here, we describe 11 new cases of TANGO2-related disease, confirming and further expanding the previously described clinical phenotype. Patients were homozygous or compound heterozygous for previously described exonic deletions or new frameshift, splice site, and missense mutations. All patients showed developmental delay with ataxia, dysarthria, intellectual disability, or signs of spastic diplegia. Of importance, we identify two subjects (aged 12 and 17 years) who have never experienced any overt episode of the catabolism-induced metabolic crises typical for the disease. Mitochondrial complex II activity was mildly reduced in patients investigated in association with crises but normal in other patients. In one deceased patient, post-mortem autopsy revealed heterotopic neurons in the cerebral white matter, indicating a possible role for TANGO2 in neuronal migration. Furthermore, we have addressed the subcellular localization of several alternative isoforms of TANGO2, none of which were mitochondrial but instead appeared to have a primarily cytoplasmic localization. Previously described aberrations in Golgi morphology were not observed in cultured skin fibroblasts.

Rare genetic variants potentially involved in ovarian hyperstimulation syndrome.

PURPOSE: We aim to investigate whether there is a genetic predisposition in women who developed ovarian hyperstimulation syndrome (OHSS) after GnRH antagonist protocol with GnRH agonist trigger and freeze-all approach. METHODS: Four patients with OHSS after GnRH agonist trigger and freeze-all approach were gathered from the worldwide patient population. These patients were analyzed through Whole Exome Sequencing. In this study known causes of OHSS were investigated and new causes present in at least two individuals were searched for. RESULTS: In the first part of the study, we evaluated the presence of mutations in genes already known to be involved in OHSS. In PGR and TP53, heterozygous alterations were detected. PGR is predicted to be involved in progesterone resistance with a recessive inheritance pattern and is, therefore, not considered as being causal. The consequences of the variant detected in TP53 currently remain unknown. In part 2 of the study, we assessed the clinical significance of variants in genes previously not linked to OHSS. We especially focused on genes with variants present in ≥ 2 patients. Two patients have variants in the FLT4 gene. Mutations in this gene are linked to hereditary lymphedema, but no link to OHSS has been described. CONCLUSIONS: Defining a genetic predisposition for OHSS is essential in view of prevention. In this study, a potential link between the FLT4 gene and OHSS has been suggested. Future functional studies are essential to define a more precise involvement of the detected variants in the development of OHSS.

New insights into the phenotype of FARS2 deficiency.

Mutations in FARS2 are known to cause dysfunction of mitochondrial translation due to deficient aminoacylation of the mitochondrial phenylalanine tRNA. Here, we report three novel mutations in FARS2 found in two patients in a compound heterozygous state. The missense mutation c.1082C>T (p.Pro361Leu) was detected in both patients. The mutations c.461C>T (p.Ala154Val) and c.521_523delTGG (p.Val174del) were each detected in one patient. We report abnormal in vitro aminoacylation assays as a functional validation of the molecular genetic findings. Based on the phenotypic data of previously reported subjects and the two subjects reported here, we conclude that FARS2 deficiency can be associated with two phenotypes: (i) an epileptic phenotype, and (ii) a spastic paraplegia phenotype.

Evidence of a wide spectrum of cardiac involvement due to ACAD9 mutations: Report on nine patients.

Acyl-CoA dehydrogenase 9 (ACAD9) is a mitochondrial protein involved in oxidative phosphorylation complex I biogenesis. This protein also exhibits acyl-CoA dehydrogenase (ACAD) activity. ACAD9-mutated patients have been reported to suffer from primarily heart, muscle, liver, and nervous system disorders. ACAD9 mutation is suspected in cases of elevated lactic acid levels combined with complex I deficiency, and confirmed by ACAD9 gene analysis. At least 18 ACAD9-mutated patients have previously been reported, usually displaying severe cardiac involvement. We retrospectively studied nine additional patients from three unrelated families with a wide spectrum of cardiac involvement between the families as well as the patients from the same families. All patients exhibited elevated lactate levels. Deleterious ACAD9 mutations were identified in all patients except one for whom it was not possible to recover DNA. To our knowledge, this is one of the first reports on isolated mild ventricular hypertrophy due to ACAD9 mutation in a family with moderate symptoms during adolescence. This report also confirms that dilated cardiomyopathy may occur in conjunction with ACAD9 mutation and that some patients may respond clinically to riboflavin treatment. Of note, several patients suffered from patent ductus arteriosus (PDA), with one exhibiting a complex congenital heart defect. It is yet unknown whether these cardiac manifestations were related to ACAD9 mutation. In conclusion, this disorder should be suspected in the presence of lactic acidosis, complex I deficiency, and any cardiac involvement, even mild.

Alteration of structure and function of ATP synthase and cytochrome c oxidase by lack of Fo-a and Cox3 subunits caused by mitochondrial DNA 9205delTA mutation.

Mutations in the MT-ATP6 gene are frequent causes of severe mitochondrial disorders. Typically, these are missense mutations, but another type is represented by the 9205delTA microdeletion, which removes the stop codon of the MT-ATP6 gene and affects the cleavage site in the MT-ATP8/MT-ATP6/MT-CO3 polycistronic transcript. This interferes with the processing of mRNAs for the Atp6 (Fo-a) subunit of ATP synthase and the Cox3 subunit of cytochrome c oxidase (COX). Two cases described so far presented with strikingly different clinical phenotypes-mild transient lactic acidosis or fatal encephalopathy. To gain more insight into the pathogenic mechanism, we prepared 9205delTA cybrids with mutation load ranging between 52 and 99% and investigated changes in the structure and function of ATP synthase and the COX. We found that 9205delTA mutation strongly reduces the levels of both Fo-a and Cox3 proteins. Lack of Fo-a alters the structure but not the content of ATP synthase, which assembles into a labile, ∼60 kDa smaller, complex retaining ATP hydrolytic activity but which is unable to synthesize ATP. In contrast, lack of Cox3 limits the biosynthesis of COX but does not alter the structure of the enzyme. Consequently, the diminished mitochondrial content of COX and non-functional ATP synthase prevent most mitochondrial ATP production. The biochemical effects caused by the 9205delTA microdeletion displayed a pronounced threshold effect above ∼90% mutation heteroplasmy. We observed a linear relationship between the decrease in subunit Fo-a or Cox3 content and the functional presentation of the defect. Therefore we conclude that the threshold effect originated from a gene-protein level.

Two siblings with homozygous pathogenic splice-site variant in mitochondrial asparaginyl-tRNA synthetase (NARS2).

A homozygous missense mutation (c.822G>C) was found in the gene encoding the mitochondrial asparaginyl-tRNA synthetase (NARS2) in two siblings born to consanguineous parents. These siblings presented with different phenotypes: one had mild intellectual disability and epilepsy in childhood, whereas the other had severe myopathy. Biochemical analysis of the oxidative phosphorylation (OXPHOS) complexes in both siblings revealed a combined complex I and IV deficiency in skeletal muscle. In-gel activity staining after blue native-polyacrylamide gel electrophoresis confirmed the decreased activity of complex I and IV, and, in addition, showed the presence of complex V subcomplexes. Considering the consanguineous descent, homozygosity mapping and whole-exome sequencing were combined revealing the presence of one single missense mutation in the shared homozygous region. The c.822G>C variant affects the 3' splice site of exon 7, leading to skipping of the whole exon 7 and a part of exon 8 in the NARS2 mRNA. In EBV-transformed lymphoblasts, a specific decrease in the amount of charged mt-tRNA(Asn) was demonstrated as compared with controls. This confirmed the pathogenic nature of the variant. To conclude, the reported variant in NARS2 results in a combined OXPHOS complex deficiency involving complex I and IV, making NARS2 a new member of disease-associated aaRS2.

Mutation of the iron-sulfur cluster assembly gene IBA57 causes fatal infantile leukodystrophy.

Leukodystrophies are a heterogeneous group of severe genetic neurodegenerative disorders. A multiple mitochondrial dysfunctions syndrome was found in an infant presenting with a progressive leukoencephalopathy. Homozygosity mapping, whole exome sequencing, and functional studies were used to define the underlying molecular defect. Respiratory chain studies in skeletal muscle isolated from the proband revealed a combined deficiency of complexes I and II. In addition, western blotting indicated lack of protein lipoylation. The combination of these findings was suggestive for a defect in the iron-sulfur (Fe/S) protein assembly pathway. SNP array identified loss of heterozygosity in large chromosomal regions, covering the NFU1 and BOLA3, and the IBA57 and ABCB10 candidate genes, in 2p15-p11.2 and 1q31.1-q42.13, respectively. A homozygous c.436C > T (p.Arg146Trp) variant was detected in IBA57 using whole exome sequencing. Complementation studies in a HeLa cell line depleted for IBA57 showed that the mutant protein with the semi-conservative amino acid exchange was unable to restore the biochemical phenotype indicating a loss-of-function mutation of IBA57. In conclusion, defects in the Fe/S protein assembly gene IBA57 can cause autosomal recessive neurodegeneration associated with progressive leukodystrophy and fatal outcome at young age. In the affected patient, the biochemical phenotype was characterized by a defect in the respiratory chain complexes I and II and a decrease in mitochondrial protein lipoylation, both resulting from impaired assembly of Fe/S clusters.

Extremely high mutation load of the mitochondrial 8993 T>G mutation in a newborn: implications for prognosis and family planning decisions.

UNLABELLED: The propositus presented with hypotonia, respiratory failure, and seizures in the newborn period and was found to have severe hyperlactacidemia and a hypertrophic heart. He carried a de novo pathogenic mutation (m.8993 T>G) in the gene encoding subunit 6 of the mitochondrial ATP synthase (MTATP6). Although the lactate concentration in serum normalized and the proband recovered after a short period at the neonatal intensive care unit, his ultimate motor and cognitive development was poor. Brain MRI at the age of 6 months showed bilaterally signal abnormalities in the caudate nucleus, putamen, thalamus, and mesencephalon. He died at the age of 9 months. The difficulty in genetic counseling in families with a maternal mitochondrial mutation disorder is emphasized. CONCLUSION: Here, we report on a neonate with the m.8993 T>G mutation and emphasize implications of mtDNA disorders on family planning decisions.

Ovarian hyperstimulation syndrome after gonadotropin-releasing hormone agonist triggering and "freeze-all": in-depth analysis of genetic predisposition.

PURPOSE: We report on the results of the whole-genome analysis performed in a patient who developed severe ovarian hyperstimulation syndrome (OHSS) following gonadotropin-releasing hormone (GnRH) agonist triggering in a "freeze-all" protocol. METHODS: A 30-year-old patient with polycystic ovary syndrome who developed severe early-onset OHSS with clinical ascites, and slight renal and hepatic dysfunction was admitted for monitoring and treatment with cabergoline and intravenous albumin. Exome sequencing to assess for any known genetic predisposition for OHSS was performed. RESULTS: No known genetic variants associated with OHSS predisposition were found. CONCLUSIONS: Case reports of severe OHSS following a "freeze-all" strategy are starting to arise, showing that OHSS has not been completely eliminated with this approach. Further studies should be conducted to confirm if such cases may be due to genetic predisposition or not.

De novo MECP2 duplications in two females with intellectual disability and unfavorable complete skewed X-inactivation.

Xq28 microduplications of MECP2 are a prominent cause of a severe syndromic form of intellectual disability (ID) in males. Females are usually unaffected through near to complete X-inactivation of the aberrant X chromosome (skewing). In rare cases, affected females have been described due to random X-inactivation. Here, we report on two female patients carrying de novo MECP2 microduplications on their fully active X chromosomes. Both patients present with ID and additional clinical features. Mono-allelic expression confirmed complete skewing of X-inactivation. Consequently, significantly enhanced MECP2 mRNA levels were observed. We hypothesize that the cause for the complete skewing is due to a more harmful mutation on the other X chromosome, thereby forcing the MECP2 duplication to become active. However, we could not unequivocally identify such a second mutation by array-CGH or exome sequencing. Our data underline that, like in males, increased MECP2 dosage in females can contribute to ID too, which should be taken into account in diagnostics.

Characterization of CoQ10 biosynthesis in fibroblasts of patients with primary and secondary CoQ10 deficiency.

Primary coenzyme Q10 (CoQ10) deficiencies are associated with mutations in genes encoding enzymes important for its biosynthesis and patients are responsive to CoQ10 supplementation. Early treatment allows better prognosis of the disease and therefore, early diagnosis is desirable. The complex phenotype and genotype and the frequent secondary CoQ10 deficiencies make it difficult to achieve a definitive diagnosis by direct quantification of CoQ10. We developed a non-radioactive methodology for the quantification of CoQ10 biosynthesis in fibroblasts that allows the identification of primary deficiencies. Fibroblasts were incubated 72 h with 28 µmol/L (2)H3-mevalonate or 1.65 mmol/L (13)C6-p-hydroxybenzoate. The newly synthesized (2)H3- and (13)C6- labelled CoQ10 were analysed by high performance liquid chromatography-tandem mass spectrometry. The mean and the reference range for (13)C6-CoQ10 and (2)H3-CoQ10 biosynthesis were 0.97 (0.83-1.1) and 0.13 (0.09-0.17) nmol/Unit of citrate synthase, respectively. We validated the methodology through the study of one patient with COQ2 mutations and six patients with CoQ10 deficiency secondary to other inborn errors of metabolism. Afterwards we investigated 16 patients' fibroblasts and nine showed decreased CoQ10 biosynthesis. Therefore, the next step is to study the COQ genes in order to reach a definitive diagnosis in these nine patients. In the patients with normal rates the deficiency is probably secondary. In conclusion, we have developed a non-invasive non-radioactive method suitable for the detection of defects in CoQ10 biosynthesis, which offers a good tool for the stratification of patients with these treatable mitochondrial diseases.

Effect of resveratrol on cultured skin fibroblasts from patients with oxidative phosphorylation defects.

Few therapeutic options are available to patients with oxidative phosphorylation disorders. Administering pharmacological agents that are able to stimulate mitochondrial biogenesis have been put forward as a possible treatment, yet the approach remains in need of thorough testing. We investigated the effect of resveratrol in an in vitro setting. Mitochondrial enzymatic activities were tested in cultured skin fibroblasts from patients harboring a nuclear defect in either complex II or complex IV (n = 11), and in fibroblasts from healthy controls (n = 11). In the latter, preincubation with resveratrol resulted in a significant increase of citrate synthase, complex II and complex IV enzyme activity. In patients with complex II or complex IV deficiency, however, activity of the deficient complex could not be substantially augmented, and response was dependent upon the residual activity. We conclude that resveratrol is not capable of normalizing oxidative phosphorylation activities in deficient cell lines.